Method and apparatus for coating articles with particulate material



2. Sheets-Sheet l R. F. STROBEL ETAL METHOD AND APPARATUS FOR COATING ARTICLES WITH PARTICULATE MATERIAL v w L D EN Neu M 5 7 w m w 5 EB 5 WV s ti l| Sept. 28, 1965 Filed Jan. 5, 1961 WQQWWNU WYQQQ Sept. 28, 1965 R. F. STROBEL ETAL 3,208,858

METHOD AND APPARATUS FOR COATING ARTICLES WITH PARTICULATE MATERIAL Filed Jan. 5, 1961 2 Sheets-Sheet 2 I NVENTORS P0195977? 5720551.

51/5 540a UND @ZMWM United States Patent 3,208,868 METHOD AND APPARATUS FOR COATING AR- TICLES WITH PARTICULATE MATERIAL Rupert F. Strobel, St. Paul, Minn., and Sven Backlund, Saltsjobaden, Sweden, assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware Filed Jan. 5, I961, Ser. No. 80,828 14 Claims. (Cl. 117-18) This invention relates to an apparatus and method for handling materials to accomplish coating of articles, and more particularly relates to apparatus and a method of handling materials for continuous coating of articles with dust-like particles without the hazard of escaping dust creating an unhealthy environment.

The coating apparatus of the invention, in its preferred embodiment, is designed so as to permit articles to be passed through a mist of dust-like coating material in a continuous manner while suspended free of contact with any part of the apparatus forming the coating zone. No seals are required at the inlet or exit ports of the coating zone; and yet escape of dust particles from the zone may be entirely prevented.

The apparatus is particularly useful for continuous coating of elongated articles such as pipes, angle irons, structural members such as those required in bridge construction, and flat articles or even small parts.

Suspension coating techniques such as spray coating or fluidized bed coating are well known, and permit the coating of articles with smooth uniformly-thick layers of thermoadhesive materials, despite irregularities such as corners, angular projections or other irregular high spots on articles. Known methods generally require that the article first be heated to a temperature at which the solid thermoadhesive particles used in coating will coalesce and fuse upon the surface of the article. These existing methods, however, suffer from several disadvantages. Ordinary spray gun application of thermoadhesive particles upon a pre-heated article creates dust problems. Employment of a fluidized bed for coating inherently limits the size of an article to be coated to the size of the fluidized bed into which the article must be dipped; and fluidized bed techniques also are not free of dust problems. Enclosure of the dust-like coated mist within a conventional spray coating chamber may serve to reduce dust problems, but continuous coating of articles moving into and out of that chamber necessitates that ports be provided for ingress and egress. Even operation of a conventional dust chamber at a reduced pressure does not entirely obviate escape of dust from the inlet and outlet ports. Further, in the conventional spray coating chamber, eddy currents are created which entrain the particulate or dust-like thermoadhesive particles and cause some surfaces of a pre-heated article to be contacted with excessive amounts of dust and other surfaces to be starved. Rotation of an elongated article as it passes through such a chamber may serve to minimize excessive or inadequate coating on various portions of the article; but to effectively accomplish this, a rather high rate of rotation for smaller-diameter elongated articles is generally required. Rotation of elongated articles such as pipe tends to introduce an additional handling problem for the pipe coater, one which becomes the more serious when smaller diameter pipe is to be coated.

This invention obviates problems such as mentioned in the foregoing paragraph and permits continuous rapid coating of articles with a uniformly-thick layer of coating material on all surfaces thereof without the necessity of rotating the article during its passage through the 3,208,868 Patented Sept. 28, 1965 coating zone. Rotation of the article thus becomes optional. The coating zone is defined by the flow of the dust material itself, and not by seals at ingress and egress ports of a coating chamber as conventionally used. In effect, the preferred embodiment of the coating apparatus of the instant invention is so designed that more than one zone of intense coating is created where particles are applied as a uniformly-thick film or layer upon all surfaces of an article passing through the apparatus.

The invention will be described with particular reference to a drawing, made a part hereof, wherein:

FIGURE 1 is a schematic representation of the association of various elements of apparatus particularly useful for coating long articles such as pipe, with the coater itself illustrated in sectional elevation;

FIGURE 2 is a schematic perspjective view of the coated partially broken away to illustrate a portion of the interior;

FIGURE 3 is a schematic illustration of a conduit system for control of the flow of dust particles through the coating chamber;

FIGURE 4 is a schematic diagrammatic view, with conduits represented by dash lines, illustrating the path of flow of particles in a continuous manner through the coating chamber (end view) and the unit (side view) for controlling the flow of the particles; and

FIGURE 5 is a schematic representation of spirally overwrapping a rotating coated article, such as a pipe, with a strip of sheet material.

Referring to FIGURE 1, the over-all operation of coating articles, e.g., pipe, with a protective resin layer, using the apparatus and materials handling method of the invention will be described. Two lengths of pipe 10 and 11, joined together by double cone coupler 12, are moved from left to right in FIGURE 1, by a drive unit A of conventional manufacture. External surfaces of the pipe to be coated are subjected to a cleaning action (e.g., sandblasting) in cleaner B, also of conventional manufacture. Just prior to being coated, the pipe passes through a heater C, where the temperature of the pipe is raised above the lowest temperature at which the dust-like or pulverulent thermoadhesive material used in coating is fused, but no higher than the temperature at which the pulverulent material decomposes on contact with the heated pipe in coater D. Where dust-like thermoadhesive particles which coalesce and thermoset rapidly on contacting a properly pre-heated pipe are employed, it is to be understood that the pre-heat temperature employed may desirably be such that adequate heat is introduced into the pipe and therefore available to effect the rapid thermosetting of such thermoadhesive particles without need for subsequent auxiliary heat. This concept will be explained further in detail below.

Directly from the heater C, the pipe is passed into the coater D, as this arrangement permits of maximum control of the temperature of the pipe (or other article to be coated) as it passes through coater D. Of course, if desired, induction heating of articles to be coated may be accomplished essentially within the area where coating with the thermoadhesive particles is done. The particular expedient employed to gain proper article heating for coalescence and even thermosetting of pulverulent thermoadhesive materials in coating may vary, as will be evident to those skilled in the art.

In coater D, particles of thermoadhesive contacting the heated pipe adhere to its surfaces and coalesce to form an essentially uniformly-thick coating which, when preferred thermosetting pulverulent particles are employed with appropriate pre-heating of the article to be coated, autogenously thermosets in situ. After leaving coater D, the pipe preferably is free of any supporting means for a short distance, the time 'for traversing that distance preferably being just sufficient for the thermosetting action of coalesced thermoadhesive particles especially designed to rapidly cure at elevated temperatures, or just suificient to allow 'non-therm'osetting particles to cool adequately to a non-tacky state after coalescence and fusion. The distance that the coated article traverse after leaving the coater without being contacted on is coated surface by any supporting element may vary; and if desired, the coated article may be passed through a cooler E prior to being contacted on its coated surface with any elements such as a support member F.

Also, if desired for the purpose of checking freedom of the coating from flaws, the coated article may be passed centrally through a circular electrode as it emerges from the coating zone of the coating chamber. For this purpose, it is suitable to use a circular conducting member having inwardly projecting conductive needles (i.e., electrodes). The coated article is grounded and passed centrally through this circular member, free of contact with the inwardly projecting needles; and the voltage applied to the circular member with its inwardly projecting electrodes is su'fiicient to form an intense electrical field with corona discharge from the needle electrodes to the article, such as pipe, passing through the field. At bare spots, pinholes or places of inadequate coating deposit, the air in the field breaks down and an arc discharge occurs. The are discharge may be used to actuate, in timed relationship, one or more of several spray guns spaced concentrically about the path of travel of the coated article. Thus a spray of patching thermoadhesive particles may be applied to repair any defect promptly, if such were ever to become necessary. I

The voltage applied to the needle electrodes may vary depending on spacing from the coated article, thickness of coating desired to be maintained, etc. Illustratively, one may use about 11,000 to 16,000 volts at a one-half inch air gap for many pipe coating operations. With a jeeper test arrangement as described, it will be evident that smudging or disruption of the hot andtherefore some what plastic coating on the article is avoided, since the coating is not contacted by any member associated with the test electrode (a deficiency of heretofore-used jeeper testing apparatus); and yet testitng of the coating is effectively accomplished promptly after its formation so that remedial measures, if ever necessary, may be accomplished promptly while the article is still hot and without the necessity of a subsequent special heating cycle.

Referring now to the sectional view of the coater in FIGURE 1 and the schematic view in FIGURE 2, the elements of structure thereof will be described. In the preferred embodiment of the invention, as illustrated in the drawing, this coater comprises two annular manifolds 13 and 14 which terminate in (i.e., are joined with) annular orifices. The annular orifice defined by ring members 15 and 16 is for annular manifold 13; and that defined by ring members 17 and 18 for annular manifold 14. Each ring member amounts to a section through a cone, with the cooperating ring members that form each annular outlet converging toward that outlet, thus in effect creating a jet outlet for material introduced in each manifold. The cone from which internal ring members 16 and 18 are taken as a section may suitably be one having an angle of approximately 70 from its sides'to its axis, whereas the cone from which external ring members 15 and 17 are taken as a section preferably is one having an angle of approximately 45 from its sides to its axis. Of course, these specific angles for the ring members may be varied considerably, the critical requirement being that the cooperating ring members forming the annular orifice for each annular manifold must converge'toward that annular outlet so as to create a jet emergence of coating material therefrom.

The ring members of each annular orifice are in spaced stacked or non-abutting relationship to each other, with the ring member from the more pointed cone and thus the one having the sharper conical angle (i.e., smaller angle from axis to sides) located with its innermost terminus pointing toward the apex of the cone of the ring member possessing the flatter conical design. As a result, an annular outlet is formed which serves to direct powdered materials emerging therefrom toward the apex of a cone, the slope of which is a compromise between the slopes of the ring members between which the material emerged. Stated another way, the angle of emergence of dust material from the annular orifices with respect to the path of articles moving through the coater is an acute angle, with its apex directed inwardly toward the coating zone. Emerging dust material from each annular orifice is thus centripetally directed inwardly of the entrance and exit openings in the coating chamber; and articles to be coated are passed essentially through the confluence of each cone of 'centripetally directed particles.

Between the annular manifolds of the coater is an enclosed chamber 19 which for convenience maybe referred to as a vacuum zone, since the pressure in this zone is maintained at a slightly reduced level (e:g., an inch of water below normal air pressure) ascompared to environmental pressure outside the coater.

Each manifold of the apparatus is fed with pulverulent material through a plurality of conduits 20, preferably equidistantly spaced annularly about each annular manifold. Three such feed conduits for each manifold serve quite satisfactorily. About the vacuum chamber (between the manifolds illustrated in the drawings) are spaced a plurality of exit ports connected with conduits 21 for removal of air within the vacuum chamber and concomitant removal of excess pulverulent material not adhered to objects passing through the chamber. For convenience of illustration in the drawing, the conduits to and from the coater are terminated a short distance from the coater.

As illustrated in FIGURES 3 and 4, the conduits 21 for removal of air and excess dust material from the vacuum chamber are merged into a single larger conduit 22 connected intermediate its ends with a'filtered vacuum pump G (graphicallyillustrated) for removal of air and collection of any dustlike particles entrained with .that air. Also, at the end of that larger conduit 22, opposite the introduction point of material from the conduits 21, is a fan H which serves also to draw air from the vacuum chamber of the coater. Just .prior to the position of the fan H in large conduit 22 is a conduit 23 for the introduction of additional pulverulent material into the system. From blower fan H, material is thrown through conduit 24 which divides into a multiple set of individual conduits 20 for feeding thepulverulent material into manifolds 13 and 14. Thus, it will be evident that the apparatus for handling the pulverulent coating materials constitutes a closed system and is capable of operation free of dust escape.

In operation, pulverulent material entrained with air and forced by the blower fan H through the several conduits 20 .into the manifolds 13 and 14 tends to evenly distribute itself in the manifolds "because of the slight reduction in air velocity that occurs at the emergence of the pulverulent material from conduits 20 into the enlarged areas constituting the manifolds. An increase in pressure develops in :the -manifolds, contributing to the even distribution of dust material therein. In essence, the airflow within each manifold is such as .to evenly distribute the resin therein before it is ejected through the annular orifices. From'each annular orifice is discharged a centripetal jet ofparticles and-the confiuence of the merging jet of particles from each annular orifice is oriented toward the vacuum chamber or zone, as well as oriented toward articles continuously passed through the coater. The reduced pressure in the vacuum chamber as compared to the external environmental air pressure causes a rush of air to flow through theinlet and exit openings or ports of the coater. The rush of air serves to entrain the pulverulent material with it and prevent or obstruct escape of pulverulent material through the ports and out of the zone of the coater itself. This is so even though the openings of the coater may be extremely large as compared to the article passing through the coater for coating. For example, coater apparatus having ports (rings 15 and 17) at each end on the order of about 8 inches diameter has been used satisfactorily in the coating of 1 inch pipe without pulverulent material escaping from the coater openings.

It should be emphasized that the continuous centripetal jet curtain of powder which emerges from the annular orifices of the manifolds converges toward the approximate vortex of the cone defined by the flow of emerging particles; thus, in effect two separate zones of coating are created within the coater illustrated in the drawing. Since, in the version illustrated in the drawing, the emerging particles from each annular orifice are directed toward each other as well as centripetally, the velocities of emergence tend to cancel as the emerging particles flow toward a meeting. This feature in combination with a slight negative pressure in the vacuum chamber serves to prevent or obstruct the particles from escaping through ports of the coater.

Many different types of solid pulverulent thermoadhesive coating materials (e.g., epoxies, phenolics, polyethylenes, etc.) may be employed in using the apparatus and materials handling system of the invention. Even thermoadhesive particles such as inorganic enamel frit particles may be employed. As used herein, thermoadhesive particles refers to particles which fuse together on heating and remain so fused after cooling. Such peraticl-es may or may not also exhibit thermosetting properties. Preferred illustrative thermoadhesive particles which also are extraordinarily rapid in their ability to thermoset or cure within seconds at elevated temperatures, are those wherein each particle comprises a uniform blend of epoxy resin, latent heat-activatible epoxyreactive hardener and accelerators or catalysts for the reaction between the epoxy resin and hardener. A useful illustrative formulation for this type of rapidly-reactive epoxy resin thermoadhesive particles, remaining stable for long periods at room temperatures or even slightly higher temperatures, is as follows: 591.4 parts of solid epoxy resin (equal parts of Epon 1001 and Epon 1002), 51 parts of isophthalyl dihydrazide, 10 parts of dicyandiamide, 2.3 parts of tris(dimethylaminomethyl) phenol, 5.3 parts of alkyl ammonium bentonite (Bentone-38), 350 parts of finely divided mica filler, and 10 parts of chrome oxide pigment. Generally, it is desired that the particles of this resin mix should pass through a 40 mesh screen; and they may be as small as 200 mesh, or even smaller (e.g., minus 325 mesh). Particles of about 80 mesh or smaller are preferred. The particles of this composition melt or fuse at about 300 F., and within a minute or so after fusing at this temperature, the mass gells and cures to thermoset infusible state. At 450 F. (a suitable pre-heat temperature for articles to be coated with this powder) the particles of this composition melt, fuse and cure to a thermoset infusible state within seconds.

The coater of the invention is particularly useful in the application of thermoadhesive particles as a uniform coating on appropriately heated articles, usually, but not necessarily, of metal. However, organic filler dusts (e.g. wood flour), or inorganic grit or other particulate material (eg talc) may be applied with thermoadhesive particles during a coating operation; or such particulate may be applied over a previously applied thermoadhesive layer by using, for example, a coating orifice (i.e., the annular coating means) of the invention. Indeed, the coater of the invention may be used to apply non-thermoadhesive particulate material over a tacky substrate article (or a tacky coating on an article), whether 6 tackification is gained by heating or is exhibited under normal room temperature conditions. Different layers of material (e.g., a thermoadhesive coating, followed by a coating of grit particles) may be applied sequentially over a base article using a series of annular coater means as described.

While the invention has been described with particular reference to coating elongated articles such as pipe, it will also be evident that the coater here described may be useful for coating discrete articles on a continuous conveyor basis or even on a piece basis such as by dipping the article within the confluence of particles in the coater. In the case of dip coating, it will be evident that one-half of the coater illustrated may be used and tilted upwardly to give a vessel for dip coating.

Also, if desired, pipe or like articles may further be wrapped with paper or other strip material after they are coated. Such wrapping is conventiently accomplished by rotating the pipe 11 during the coating operation and spirally applying the strip material 25 as the rotating coated pipe emerges from the coater, as illustrated in FIGURE 5.

Sometimes vibration of the coater during the coating operation serves as a desirable adjunct to air and powder flow to obviate clogging of the apparatus.

As will be evident to those skilled in the art, the coater of the instant invention may comprise a single or a plurality of manifolds and accompanying annular orifices in linear arrangement, with vacuum zones as described associated therewith. It will also be evident that the essential attributes of annular orifices as described may be gained by elliptical as well as circular members. Distorted shapes of myraid configuration may be designed to give equivalent results; and such, of course, is comprehended by the claims.

That which is claimed is:

1. Apparatus for coating articles comprising a chamber having a port therein defined by substantially annular means to discharge a substantially continuous centripetal jet curtain of solid particles upon an article passed through said port in the confluence of said discharged particles, manifold means for essentially uniformly distributing said particles prior to discharge of the same through said annular means, a plurality of supply conduits for delivering particles to said manifold means, a plurality of exhaust conduits for removing air from said chamber as well as air-entrained particles exceeding the quantity of particles employed in the coating operation Within said chamber, said exhaust conduits being connected to exhaust means suflicient to create a reduced pressure within said chamber as compared to environ mental pressure outside said chamber, thereby to effect a flow of external air through said port into said chamber for obstructing escape of said discharged particles outwardly of said port, and means connecting said exhaust conduits to said supply conduits to thereby provide an enclosed continuous path permitting flow of particles into and out of said chamber.

2. Apparatus for coating articles comprising a cylindrical chamber having a port at one end thereof defined by substantially annular means to discharge a substantially continuous centripetal jet curtain of solid particles upon articles in the confluence of said discharged particles, manifold means for essentially uniformly distributing said particles prior to discharge of the same through said annular means, a plurality of supply conduits for delivering particles to said manifold means, exhaust means to create a zone of reduced pressure inwardly of said port, and means to move articles through said discharged particles for coating.

3. Apparatus for coating articles comprising a chamber having entrance and exit ports, each defined by substantially annular means for discharging a substantially continuous centripetal jet curtain of solid particles inwardly into said chamber, a plurality of supply conduits for feeding particles to, each of said substantially annular mean said supp y nduit eing conn e t s anial y equ distant annu a spa n to ai s bs a y nnular mean nd a Withdr a m an eff i to n.- t n y xcess of d scha g d par icles not mpl y d r coating an article as it passes through said chamber and remove said entrained particles from said chamber.

4. Apparatus for coating articles comprising a substantially cylindrical chamber having an entrance port and an exit port lineally aligned, each of said ports being defined y ubs a ally n lar mea s f rg g a substantially continuous centripetal jet curtain of solid particles inwardly into said chamber, a plurality of supply on uits o f ed n Pa cl s to eac f s d s tially annular means, said supply conduits being connected at substantially equidistant annular spacing to said substantially annular means, and each of said ports being associated with means to create a reduced pressure inwardly of the same within said chamber as compared to environmental pressure outside said chamber, thereby to effect a flow of external air through the same into said chamber for obstructing escape of said discharged particles outwardly from said ports.

5. Apparatus for coating articles comprising a chamber having an entrance port and an exit port, each of said ports being defined by substantially annular means for discharging a substantially continuous centripetal jet curtain of solid particles conically into said chamber, a plurality of supply conduits for. feeding particles to each of said substantially annular means, said supply conduits being connected at substantially equidistant annular spacing to said substantially annular means, means to create a reduced pressure within said chamber as compared to environmental pressure outside said chamber and thereby elfect a flow of external air through each of said ports into said cham ber for obstructing escape of said discharged particles outwardly from said ports, and means to move articles essentially through the confluence of the particles discharged by the annular means at each of said ports.

6. Apparatus for coating articles comprising a chamber having entrance and exit ports, each defined by substantially annular means for discharging a substantially continuous centripetal jet curtain of particlesupon articles passed through each said port in the confluence of said said discharge particles, a plurality of suPPly conduits for feeding particles to each of said substantially annular means, said supply conduits being connected at substantially equidistant annular spacing to said substantially annular means, means to move articles through said discharged particles for coating, air Withdrawal means effective to entrain and remove any excess of said discharged particles within said chamber not employed for coating an article as it passes through said chamber, and means to recirculate said removed particles to said coating chamber. 7

7. Apparatus for coating articles comprising a chamber having a port therein defined by substantially annular means for discharging a substantially continuous centripetal jet curtain of solid particles upon an article passed through said port in the confluence of said discharged particles, a plurality of supply conduits for feeding particles to each of said substantially annular means, said supply conduits being connected at substantially equidistant annular spacing to said substantially annular means, means to create a reduced pressure within said chamber as compared to environmental pressure outside said chamber andthereby effect a flow of external air through said port into said chamber for obstructing escape of said discharged particles outwardly from said port, and air flow means to remove any excess of discharged particles from said chamber and re-introduce them through said annular means.

8. The method of providing an article with a thermoset adhesive coating comprising the steps of heating the article to a temperature above the softening temperature of a solid thermosettable thermoadhesive material in particulate form, forming a substantially conical centripetal jet curtain of particles of said solid thermosettable thermoadhesive material in a gaseous stream, passing the article along the major axis of the conical centripetal jet curtain of solid thermosettable thermoadhesive particles and through the confluence of said centripetal jet curtain of said particles while the temperature of the article is above the softening point of said particles, whereby the particles of said solid thermosettable thermoadhesive material striking said heated article fuse togetherinto a continuous coating, and heat-curing said coating to a the m s c nd i n.-

9. The method of coating an elongated article comprising the steps of heating the article to a temperature above the softening temperature of a solid thermoadhesive material to be applied as a layer upon the article, forming a substantially conical centripetal jet curtain of particles of said thermoadhesive material in a gaseous stream, passing the article While rotating the same along the major axis of said conical centripetal jet curtain of said particles and through the confluence of said centripetal jet curtain of said particles while the temperature of the article is above the softening point of said particles, thereby to effect deposit and fusion of thermoadhesive particles upon surfaces of said heated article, and then spirally overwrapping said rotating coated article with a strip of sheet material.

10. The method of coating non-thermoadhesive particulate material on an article comprising the steps of tackifying at least those surface portions of said article to be subsequently coated with said non-thermoadhesive particulate material, forming a substantially conical centripetal jet curtain of said non-thermoadhesive particulate material in a gaseous stream, and passing the article along the major axis of said conical centripetal jet curtain of said non-thermoadhesive particulate material and through the confluence of said centripetal jet curtain of said nonthermoadhesive particulate material for coating.

11. The method of coating an elongated article comprising the steps of heating the article to a temperature above the softening temperature of particles of solid thermoadhesive material to be applied as a layer upon the article, forming a substantially conical centripetal jet curtain of said particles of thermoadhesive material in a gaseous stream, passing the article in aligned longitudinal condition along the major axis of said conical centripetal jet curtain of said particles and through the confluence of said centripetal jet curtain of said particles while the temperature of the article is above the softening temperature of said particles, thereby to effect deposit and fusion of said thermoadhesive particles upon surfaces of said heated article.

12. The method of providing a metal pipe with a continuous resinous coating formed from discrete solid thermoadhesive particles, comprising the steps of heating the article to a temperature above the softening tempera,- ture of said solid thermoadhesive particles, forming at least one substantially conical centripetal jet curtain of airborne particles of said solid thermoadhesive material, passing said pipe in aligned longitudinal condition along the major axis of said conical centripetal jet curtain of said particles and through the confluence of said centripetal jet curtain while maintaining the temperature of said pipe above the softening point of said particles and while simultaneously exhausting from the vicinity of said pipe those particles not adhering to said pipe, said conditions of movement of said pipe through said curtain of particles being such as to cause sufiicent of said particles to adhere to said pipe to form a continuous coalesced resinous. coating thereon.

13. Apparatus for coating metal pipe comprising an elongated coating chamber open at ,opposite ends for movement of pipe longitudinally therethrough, exhaust means connected to said chamber through conduit connections intermediate the ends of said chamber, said exhaust means being sufficient to efiect an inward flow of external air through the openings at opposite ends of said chamber, a plurality of supply condutis with annularlyflared discharged openings connected to said apparatus annularly about at least one open end of said coating chamber, whereby air-entrained solid thermoadhesive particles forced through said supply conduits are discharged in a substantially continuous annular converging centripetal jet curtain into said chamber and toward said exhaust conduit connections, blower means to force airentrained solid thermoadhesive particles through supply conduits of said apparatus, means externally to said chamber to effect movement of pipe through one open end of said chamber and out the other without causing contact between coated surfaces of said pipe and said chamber, and means to re-introduce into said chamber through said supply conduits those thermoadhesive particles removed from said chamber by said exhaust means.

14. Apparatus for coating metal pipe comprising an elongated coating chamber open at opposite ends for movement of pipe longitudinally therethrough, exhaust means connected to said chamber through conduit connections intermediate the ends of said chamber, said exhaust means being suflicient to effect an inward flow of external air through the openings at opposite ends of said chamber, a plurality of supply conduits with annularlyflared discharge openings connected to said apparatus annularly about one open end of said chamber, whereby air-entrained solid thermoadhesive particles forced through said supply conduits are discharged in a substantially continuous annular converging centripetal jet curtain into said chamber and toward said exhaust conduit connections, plural supply conduits connected to said apparatus annularly about the other open end of said chamber for feeding air-entrained solid thermoadhesive particles for discharge into said chamber, blower means to force air-entrained solid thermoadhesive particles through supply conduits of said apparatus, means externally to said chamber to effect movement of pipe through one open end of said chamber and out the other without causing contact between coated surfaces of said pipe and said chamber, and means to reintroduce into said chamber through said supply conduits those thermoadhesive particles removed from said chamber by said exhaust means.

References Cited by the Examiner UNITED STATES PATENTS 995,998 6/11 Bradley 1l8309 1,263,858 4/18 Cole ll8309 X 1,682,823 9/28 Barord 118--63 1,952,502 3/34 Kinkead 117180 X 2,053,307 9/36 Wilson.

2,287,825 6/ 42 Postlewaite.

2,380,968 8/45 Kimmig et a1 118-9 2,573,815 11/51 Smith.

2,880,109 3/59 Current et al.

2,988,641 6/61 Gough 118-9 XR 3,004,861 10/61 Davis 11721 XR 3,065,350 11/62 Graner 1l89 X 3,108,022 10/63 Church 1l8--429 X RICHARD D. NEVIUS, Primary Examiner. 

12. THE METHOD OF PROVIDING A METAL PIPE WITH A CONTINUOUS RESINOUS COATNG FORMED FROM DISCRETE SOLID THERMOADHESIVE PARTICLES, COMPRISING THE STEPS OF HEATING THE ARTICLE TO A TEMPERATURE ABOVE THE SOFTENING TEMPERATURE OF SAID SOLID THERMOADESHIVE PARTICLES, FORMING AT LEAST ONE SUBSTNATIALLY CONICAL CENTRIPETAL JET CURTAIN OF AIRBORNE PARTICLES OF SAID SOLID THERMOADHESIVE MATERIAL, PASSING SAID PIPE IN ALIGNED LONGITUDINAL CONDITION ALONG THE MAJOR AXIS OF SAID CONINCAL CENTRIPETAL JET CURTAIN OF SAID PARTICLES AND THROUGH THE CONFLUENCE OF SAID CENTRIPETAL JET CURTAIN WHILE MAINTAINING THE TEMPERATURE OF SAID PIPE ABOVE THE SOFTENING POINT OF SAID PARTICLES AND WHILE SIMULTANEOUSLY EXHAUSTING FROM THE VICINITY OF SAID PIPE THOSE PARTICLES NOT ADHERING TO SAID PIPE, SAID CONDITIONS OF MOVEMENT OF SAID PIPE THROUGH SAID CURTAIN OF PARTICLES BEING SUCH AS TO CAUSE SUFFICENT OF SAID PARTICLES TO ADHERE TO SAID PIPE TO FORM A CONTINUOUS COALESCED RESINOUS COATING THEREON. 